Move linked list operations into a class

From @bfolts

Create a double linked list class and move the list manipulation logic into that class that will make the code easier to test in isolation, easier to read, and we could re-use the double linked list eventually if we ever wanted to in another class.

You are going to have a lot of linked list operations you are performing often. You may want to consider creating a DoubleLinkedList class that has an appendTail and remove operation. It should make this code easier to read and make it easier to test the operations of your DoubleLinkedList.

You can utilize a generic and have DLLNode where T is Subscription for you. If you then create a DoubleLinkedList class I think this could help with testing and readability, in your priority matrix you can then store an instance of a DoubleLinkedList, all of your linked list operations you can then focus on outside of your other classes.